Turnover control in photoreceptor membrane is critical for normal maintenance of its basic photon catching mechanism as well as for appropriately adapting it to the wide range of natural ambient light conditions under which it must function optimally. Major external control is mediated by light, darkness and their normal daily alternations. Endogenous control is mediated by genetic, neural and endocrine factors. We plan to use a particularly favorable receptor, the rhabdom membrane of decapod crustaceans as a model. This system lacks pigmented epithelium so the whole synthetic and degradative process is more out in the open than in vertebrate retinas. In addition membrane turnover rates may be so high that 100% increases occur in hours rather than days as in vertebrates. We propose a quantitative study using transmission and freeze fracture electron microscopy as well as light microscopy to measure effects of changing various control factors. FFEM permits monitoring the density distribution and spacing of protoplasmic particles which represent aggregates of 3-4 rhodopsin molecules. TEM and LM allow rhabdom shape, size, rhabdom banding, microvillus length, diameter, number, pattern, and regularity to be measured. Similarly Golgi, endoplasmic reticulum and lysosome like bodies, clearly involved in supply and removal of membrane components, will be quantitatively studied. Preliminary experiments demonstrate that in crayfish darkness sustained for more than a week disrupts microvillus regularity and decreases to about half rhodopsin particle density. In the rock crab grapsus maximum dark adaptation increases photoreceptor membrane area by a factor of nearly 20X over the fully light adapted state. However, this factor varies in a quasi sinusoidal manner with time of day. Hence an endogenous cyclic control probably neurohumoral is modifying direct effects of light and dark per se on membrane turnover. Neuroendocrine techniques will be used to test this hypothesis.